As cutting tools, cemented carbides (WC—Co alloys or WC—CO alloys further containing carbonitride of Ti (titanium), Ta (tantalum), or Nb (niobium) have been used. However, with recent increases in cutting speed, there has been increased the ratio of use of hard alloy tools each including a substrate composed of a cemented carbide, cermet, a sintered cubic boron nitride compact, or an alumina or silicon nitride ceramic, and at least one coating formed on the surface of the substrate by a CVD (Chemical Vapor Deposition) method of a PVD (Physical Vapor Deposition) method using a compound of at least one first element selected from the group IVa elements, Va elements, VIa elements in the periodic table, Al (aluminum), Si, and B and at least one second element selected from B, C, N, and O (when the first element is B alone, the second element is other than B), the thickness of the coating being 3 to 20 μm.
Such cutting tools have a rake face in contact with cutting chips of a work piece in a cutting work and a flank face in contact with the work piece, and a portion (and the vicinity thereof) corresponding to the edge at the crossing of the rake face and the flank face is referred to as an “edge line”.
The cutting speed has recently increased for further improving the cutting efficiency, and accordingly, such cutting tools have been required to have higher wear resistance. However, toughness is decreased by the requirement for higher wear resistance, and thus it has been required to satisfy both high wear resistance and high toughness.
As an attempt to comply with the requirement, for example, Japanese Unexamined Patent Application Publication No. 5-177411 (Patent Document 1) pays attention to residual tensile stress a coating layer which occurs when the coating layer is formed on the substrate at a high temperature by the chemical vapor deposition method (CVD method) and then cooled to room temperature, and proposes a measure for overcoming a decrease in toughness of a cutting tool due to the residual tensile stress. Namely, the tensile stress occurs due to a difference between the thermal expansion coefficients of the substrate and the coating layer. The measure uses a method of improving toughness (resistance to fracture) while maintaining high wear resistance, in which a first coating layer having such tensile stress is first formed on the substrate, and a second coating layer having compressive stress is formed on the first coating layer after a predetermined crack is formed in the first coating layer.
Japanese Unexamined Patent Application Publication No. 5-177412 (Patent Document 2) uses an approach different from the above-described measure with attention to tensile stress of the coating layer as described above, and proposes a method in which an inner coating layer having tensile stress is formed on a hard ceramic substrate, and an outer coating layer having compressive stress is formed on the inner coating layer. Further, Japanese Unexamined Patent Application Publication No. 5-177413 (Patent Document 3) proposes a cutting tool having the same constitution as in Patent Document 2 including a cermet substrate.
On the other hand, Japanese Unexamined Patent Application Publication No. 6-055311 (Patent Document 4) proposes a cutting tool including a cemented carbide substrate and a hard coating layer formed thereon by the chemical vapor deposition method, wherein the tensile stress of the hard coating layer in a rake portion is substantially removed while holding the tensile stress of the hard coating layer in a flank portion.
Further, Patent Publication No. 3087465 (Japanese Unexamined Patent Application Publication No. 6-079502, Patent Document 5) proposes a cutting tool including a titanium carbonitride-based cermet substrate and a hard coating layer formed on the surface thereof and having a compressive stress distribution substantially the same over the entirety of a cutting edge, wherein the hard coating layer is subjected to shot blasting so that the compressive stress possessed by a rake portion is 49 MPa or more higher than that possessed by a flank portion.
In each of the above proposals, both the toughness and the wear resistance can be improved to some extent, but a cutting tool is required to have higher performance under current circumstances surrounding cutting tools. Therefore, the development of a cutting tool satisfying such performance is demanded.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 5-177411    Patent Document 2: Japanese Unexamined Patent Application Publication No. 5-177412    Patent Document 3: Japanese Unexamined Patent Application Publication No. 5-177413    Patent Document 4: Japanese Unexamined Patent Application Publication No. 6-055311    Patent Document 5: Patent Publication NO. 3087465 (Japanese Unexamined Patent Application Publication No. 6-079502)